Meson needs the "system = xyz" line when cross-compiling. We are already
adding a "system = 'windows'" for the MinGW cross-compilation case here,
so let's add a "system = 'linux'" now for Linux hosts, too.

Message-Id: <20200823111757.72002-2-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>